Claims this article. A study carried out by Which? has found that many leading brands of toddler milk are less nutritious than cows milk. Although most claim to have added health benefits for your toddler it seems you are better off giving them cows milk instead. Many brands of toddler milk or follow on milk as it is often marketing had less calcium and more sugar than cows milk.
Personally I’m unsure who actually buys follow on milk. I switched my daughter to cows milk as soon as I was able to as the formula milk was costing me a fortune and I aim to do the same with my son. I suppose some people might have to use alternatives if their child has a a dairy intolerance but otherwise I think it makes sense for the whole family to drink the same milk.